Fix to-device messages not being sent to MSC3202-enabled appservices

[turt2live]

The field name was simply incorrect, leading to errors.

Fixes turt2live/matrix-bot-sdk#208 (mostly)

Example error:

element-dev-stack-eledev-synapse-1      | 2022-07-09 01:52:01,578 - synapse.metrics.background_process_metrics - 242 - ERROR - as-sender-encrypted_appservices-4 - Background process 'as-sender-encrypted_appservices' threw an exception
element-dev-stack-eledev-synapse-1      | Traceback (most recent call last):
element-dev-stack-eledev-synapse-1      |   File "/usr/local/lib/python3.9/site-packages/twisted/internet/defer.py", line 1660, in _inlineCallbacks
element-dev-stack-eledev-synapse-1      |     result = current_context.run(gen.send, result)
element-dev-stack-eledev-synapse-1      | StopIteration: ({('@crypto_bot_user:localhost', 'URSRMAKHSN'): [{'content': {'algorithm': 'm.olm.v1.curve25519-aes-sha2', 'sender_key': 'GBc4Vu8RoVOy14212/sMAxA6oJ+ii6xsTdc9oaoY6j8', 'ciphertext': {'LU38crr35sqw4Sbm9gy3BA6dCfaxZWV4KomAEnJgmUU': {'type': 0, 'body': 'redacted'}}}, 'type': 'm.room.encrypted', 'sender': '@travis:localhost'}]}, 5379)
element-dev-stack-eledev-synapse-1      | 
element-dev-stack-eledev-synapse-1      | During handling of the above exception, another exception occurred:
element-dev-stack-eledev-synapse-1      | 
element-dev-stack-eledev-synapse-1      | Traceback (most recent call last):
element-dev-stack-eledev-synapse-1      |   File "/usr/local/lib/python3.9/site-packages/synapse/metrics/background_process_metrics.py", line 240, in run
element-dev-stack-eledev-synapse-1      |     return await func(*args, **kwargs)
element-dev-stack-eledev-synapse-1      |   File "/usr/local/lib/python3.9/site-packages/synapse/appservice/scheduler.py", line 274, in _send_request
element-dev-stack-eledev-synapse-1      |     ) = await self._compute_msc3202_otk_counts_and_fallback_keys(
element-dev-stack-eledev-synapse-1      |   File "/usr/local/lib/python3.9/site-packages/synapse/appservice/scheduler.py", line 333, in _compute_msc3202_otk_counts_and_fallback_keys
element-dev-stack-eledev-synapse-1      |     users.update(device_message["user_id"] for device_message in to_device_messages)
element-dev-stack-eledev-synapse-1      |   File "/usr/local/lib/python3.9/site-packages/synapse/appservice/scheduler.py", line 333, in <genexpr>
element-dev-stack-eledev-synapse-1      |     users.update(device_message["user_id"] for device_message in to_device_messages)
element-dev-stack-eledev-synapse-1      | KeyError: 'user_id'

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[turt2live]

hmm, as of writing github is missing a whole 2 commits from this PR for some reason. They're on the branch: develop...travis/fix-enc-appservices

Edit: and of course they magically surfaced...

[squahtx]

The bug has existed since the implementation of MSC3202 in #11617.
The to_user_id field name comes from MSC2409, implemented in #11215.

[squahtx]

lgtm!